Respiratory syncytial virus is the most common cause of lower respiratory tract disease in young children worldwide. The F glycoprotein of this virus is one of the targets of a protective immune response; however, despite significant conservation in the F glycoprotein and the development of anti-F antibodies following natural infection, immunity is incomplete. Repeated infections with viruses of the homologous or heterologous subtype occur despite high levels of neutralizing antibody. In addition, antibodies that neutralize extracellular virus may not be able to inhibit fusion, allowing cell-to-cell spread of virus. These findings suggest that intratypic antigenic differences in the F glycoprotein might be able to convey an epidemiological advantage during the course of natural infection. In order to more completely characterize the role of the hydrophobic amino terminus of F1 in virus mediated membrane fusion, we are developing monoclonal antibodies to this region of the fusion glycoprotein. These reagents will be used to characterize the expression of fusion related sequences over time, compare viruses that make small or large plaques in tissue culture.